In order to operate LED luminaires, generally power-factor-corrected switched mode power supplies are used. Since an LED string, in particular a dimmable LED string, does not represent a constant load, these switched mode power supplies are generally subjected to closed-loop control. For this, monitoring of the output voltage of the switched mode power supply is generally carried out. This output voltage is used as controlled variable. In order to achieve a favorable power factor and a low level of feedback onto the electrical grid, this closed-loop control generally has a very high time constant. Thus, the controller generally operates at <20 Hz, for example. Closed-loop control which implements changes in the power factor during an entire oscillation of the line voltage would result in a less favorable power factor and thus in an increased level of feedback onto the electrical grid.
In particular in the case of rapid changes in load, for example in the case of sudden connection, disconnection or dimming of an LED string, the conventional slow closed-loop control cannot follow. This results in a marked fluctuation in the output voltage of the switched mode power supply.
For example, WO 2011/045372 A1 discloses an operating circuit for operating LED modules. A DC voltage is generated from an AC voltage by means of power factor correction in said document. In particular in the case of rapid changes in load, however, a fluctuation in the output voltage results.
The present invention is based on the object of providing a method and a circuit arrangement which ensure safe and fault-free operation of a rapidly changing load.